1. Field of the Invention
The invention relates in general to abrasive core particles metallurgically bonded to a metal deposit, and, in particular, to core abrasive particles metallurgically bonded to encapsulating coatings comprising ruthenium, rhenium, osmium, alloys, and mixtures thereof, and to composites and abrasive compacts containing such coated abrasive particles, and to methods of preparing such coated abrasive particles, composites and abrasive compacts.
2. Description of the Prior Art
Abrasive particles have long been embedded in various matrix/binder materials for use as cutting tools, grinding wheels, and the like. Abrasive particles have also been utilized to lend hardness to articles where no abrasion is involved. Difficulty had been experienced in retaining the abrasive particles in the matrix/binder materials. Various expedients had been proposed to improve the retention and/or wetting of the abrasive core particles in the matrix/binder materials. It is well known to coat abrasive core particles with metal coatings so as to, inter alia, improve the retention of the abrasive core particles in a matrix/binder material. Typically, such metal coatings had relied on the formation of a chemical bond with the abrasive core particles for their retention properties. For example, with carbon containing abrasive core particles such as diamond or metal carbides, metal coatings had been selected for their ability to form carbides, or for their ability to wet carbon at high temperatures. Titanium, chromium, zirconium, and tungsten, for example, react to form a carbide with the carbon in diamond or carbide, which results in the formation of a chemical bond between the carbide forming element and a diamond or carbide abrasive core particle. Metals that form chemical bonds in this manner are typically described as active metals. Generally, the active metals also exhibited good adhesion reception with respect to the common metallic, resin, ceramic, or the like matrix/binder materials. It had previously been proposed that as an alternative to active metals and carbide formers, non-carbide forming cubic metals, such as cobalt, nickel, palladium, and platinum could be used to improve wettability and retention of diamonds and carbide materials. Such cubic materials are isostructural with cubic metal carbides and diamond. Such cubic materials also have high solubilites for carbon at elevated temperatures (typically at temperatures above one half the melting point of the cubic metals). A combination of metallurgical and mechanical bonds is typically formed between such cubic metals and diamond, cubic metal carbides, borides, nitrides, oxides, and the like. Other metals, such as copper, have also been used solely to promote wetting while only providing a mechanical bond to the abrasive grains.
Other prior proposed expedients for improving the retention of abrasive particles had included, for example, etching or otherwise modifying the surface of the abrasive particle to improve mechanical bonding.
Abrasive core particles generally comprise, for example, diamond, cubic metal carbides, cubic metal borides, cubic metal nitrides, cubic metal oxides, other ceramics, and the like, of various elements. Abrasive core particles, whether in compact or discrete form, are generally used to form tools, wear components, hardfacing alloys, and the like. Earth or rock drilling and boring tools such as are used, for example, in the mining and oil production fields are particularly benefited from the present invention. Metal working tools also benefit from the present invention. Typically, coated abrasive particles made according to the present invention are mounted to a tool holder, the nature of which is dictated by the intended use. Typical mounting procedures include, for example, sintering, brazing, casting, plasma spraying, thermal spraying, or the like to form coatings or compacts. Single particles can be mounted, if desired. Often, the particles are formed into a composite of a desired configuration, and the preformed composite is then mounted to the tool holder. For some applications a binder/matrix that incorporates the abrasive particles is formed to the desired configuration in situ on the tool holder.
Reliance on chemical bonding for abrasive core particle retention limits the elements that can be employed for retention purposes. Chemical bonds form interfacial materials at the boundary between the surface of the particle and the overlaying deposit. Such materials are generally not ductile so the chemical bonds are susceptible to being broken by thermal and mechanical shock, which undesirably reduces the particle retentive capacity of the coating system.
Those concerned with these problems recognize the need for improvement.
A preferred embodiment of the coated abrasive core particles according to the present invention comprises a deposit formed in situ on the surface of an abrasive core particle, which deposit forms a metallurgical bond, rather than a chemical bond with the abrasive core particle. The deposit comprises a non carbide forming hexagonal metal, which has a melting point above about 1,000 degrees centigrade, and forms a metallurgical bond with the abrasive particle. The metals that meet these criteria are ruthenium, rhenium and osmium. Cobalt, which had previously been proposed for use in bonding diamonds, has a hexagonal polymorph, however, the stable structure above 450 degrees centigrade is the cubic phase. In this cobalt is analogous to nickel.
The use of a hexagonal refractory metal from the group ruthenium, rhenium and osmium unexpectedly results in stabilization of the diamond, metal carbide, boride, nitride, oxide, or the like, structure, even though the hexagonal lattice is not isomorphous with the cubic diamond structure. This is contrary to what previous understandings of these materials generally suggested. For example, cubic metals are isostructural with diamond, while hexagonal metals are isostructural with graphite. This would seem to suggest that the hexagonal metal would destabilize cubic structures such as diamond, carbide or other abrasives. Also, cubic metals such as nickel, and the high temperature allotrope of cobalt, tend to be more ductile and have higher solubility for carbon than do the hexagonal metals. This also would seem to suggest away from the use of refractory hexagonal metals to retain diamonds, carbides and other abrasives in binder/matrix materials. Unexpectedly, it has been found that these hexagonal refractory metals are particularly effective in retaining diamonds, metallic carbides, borides, nitrides, oxides, and the like abrasive particles. The high temperature capabilities, strengths and other inherent characteristics of the refractory metals contribute substantially to the retention of the abrasive particles, and to other desirable properties of the abrasive loaded binder/matrix articles that are made with such abrasive particles.
The use of hexagonal refractory metals to retain cubic nitride, boride and oxide abrasive particles provides very satisfactory results. Without wishing to be bound by any theory, the following is believed to be one possible explanation for this. The borides, nitrides (if formed), and oxides of ruthenium, rhenium and osmium are much less stable than the borides, nitrides and oxides of which the abrasive particles are formed. The abrasive borides, nitrides or oxides (non-carbides) are slightly soluble in the hexagonal refractory metal. Thus, a small amount of these non-carbides dissolves in the hexagonal refractory metal without the formation of brittle intermetallics, or the like. This provides a very good metallurgical bond.
Metallurgical bonds are formed between different materials when one material is soluble-in the other, without any significant chemical reaction. That is, a metallurgical bond is formed when the materials form solid solutions at the interface between them without forming intermediate compounds. Metallurgical bonds or solid solutions between metals and abrasive core particles tend to be more ductile than chemical bonds. Metallurgical bonds are thus generally able to withstand more or different mechanical and thermal shocks than chemical bonds. For example, ruthenium and rhenium form solid solutions with carbon, so deposits of these metals on diamonds form metal-carbon solid solutions at the interface with the diamond. Heat treating the composite particles, followed by rapid cooling tends to enhance the strength of metallurgical bonds.
In general, the composite abrasive particles according to the present invention comprise at least abrasive core particles that have deposits, preferably encapsulating deposits, of a metal which forms a metallurgical bond with one or more of the constituents of the abrasive core particles. Such constituents include, for example, carbon, borides, nitrides, oxides, and the like. The abrasive core particles generally exhibit a hardness on Mohs scale of hardness in excess of about 7. Quartz exhibits a hardness of 7, and diamond is 10 on this scale.
Conventional procedures can be used to form the deposit in situ on the surfaces of the abrasive core particle. Such procedures include, for example, chemical vapor deposition, electroless deposition, physical vapor deposition, sputtering, salt deposition, and the like. The deposits, generally layers, are generally formed in situ from ions, atoms or nano-particles that build up on the surfaces of the abrasive core particle to form the desired deposit. This is to be distinguished from a solid object that is formed at some other location and applied as a separate object to the surface of the particle. Preferably, the two-phase composite abrasive particle is heated to increase the thickness and nature of the interlayer. In general, the interlayer is a solid solution. Rapid cooling generally tends to retain the thickness and nature of the metallurgical bond.
The strength of the metallurgical bond is preferably enhanced by pre-treating the surface of the abrasive core particle. Such pre-treatments include, for example, the application of vacuum and heat to drive of volatiles, chemical or physical etching, or the like. Broadly, these procedures involve the cleaning of the surface. Ceramic abrasive core particles can be, for example, reduced or oxidized to enrich the species on the surface. As used herein, unless otherwise indicated, the word xe2x80x9ccleaningxe2x80x9d is intended to include all such surface enhancement operations.
The preferred deposit morphology is a continuous encapsulating deposit of approximately uniform thickness. Other deposit morphologies such as crystalline, dendritic, discontinuous, or the like can be employed, if desired. The abrasive core particles can be of any shape, for example, spherical, jagged irregular, regular crystalline, fibrous, flat flake, or the like. Typically, any production run of particles will exhibit a statistical distribution of sizes and shapes around some predetermined norm.
Various optional deposits can be applied to the composite abrasive particle for various purposes. Bonding aids can be deposited in situ over the metallurgically bonded deposit. Such bonding aids are selected so that they adhere well to both the metallurgically bonded deposit and a matrix/binder into which the composite particles are to be dispersed. Such bonding aids typically include, for example, metals such as iron, iron based alloys, nickel, nickel based alloys, and cobalt and cobalt based alloys, mixtures thereof, alloys thereof, and the like. Bonding aids can be formed in situ using, for example, the procedures described for the formation of the metallurgically bonded coatings. Bonding aids are generally applied as an overcoating on the metallurgically bonded deposit, and they are capable of wetting the surface to which they are applied. The resulting composite particles are still conveniently described as xe2x80x9ctwo-phase composite particlesxe2x80x9d because the overcoatings are generally alloyed with the metallurgically bonded deposit.
Particularly when the composite abrasive particles are to be used in fabrication procedures such as brazing, plasma spraying, or the like where the binder/matrix material is caused to melt, a diffusion limiting barrier can be applied over the metallurgically bonded deposit. Such a diffusion barrier limits the intermingling of the metallurgically bonded deposit with the molten matrix so as to protect the metallurgical bond between the deposit and the abrasive core particle. Such diffusion barriers are known and include, for example, titanium carbide, chromium carbide, and the like. Such diffusion barriers are preferably very thin, in the order of approximately a few hundred Angstroms thick. They function to prevent the molten binder/matrix material from disrupting the metallurgical bond, not to contribute significantly to the abrasive qualities of the composite. Thus, a diamond abrasive core particle coated with an encapsulating layer of formed in situ metallurgically bonded rhenium, and a flash coating of aluminum oxide, exhibits abrasive qualities primarily because of the core particle, not significantly because of the flash coating. The flash coating protects the metallurgical bond by preventing the molten binder/matrix material from disrupting it.
Generally, the applications where abrasive particles are used entail the use of some binder/matrix material in addition to the metallurgically bonded deposit. In some applications, however, the composite abrasive particles are sintered, compressed or fused so that the metallurgically bonded deposits serve to bond adjacent particles together into a compact without the addition of any additional binder/matrix material.
The composite abrasive materials disclosed here are described for the sake of convenient reference as abrasives, however, it will be understood by those skilled in the art that these composite materials also find utility in non-abrasive applications. For example, these materials find application where the characteristic of hardness, or some other characteristic, is desired, without regard to whether abrasion is involved. The term xe2x80x9cabrasivexe2x80x9d is intended to include all such hard core particles whether they are used or are capable of being used as abrasives.
The composite abrasive materials disclosed here are typically formed as discrete particles, however, if desired, particularly with very fine particles, agglomerates of a few (less than approximately 6 particles) fine abrasive core particles can be coated with metallurgical bond forming material. Such small agglomerates are intended to be included within the phrase, xe2x80x9cabrasive core particlesxe2x80x9d.
Matrix/binder materials that are generally suitable for use with composite abrasive particles according to the present invention include, for example, metals such as iron, iron based alloys, nickel, nickel based alloys, cobalt and cobalt based alloys, copper, copper based alloys, chromium based alloys, mixtures thereof, alloys thereof, and the like. Brazing materials such as, for example, gold, silver, copper, nickel, gallium, tin, mixtures and alloys thereof are also generally suitable for such use. Vitreous matrix/binders can also be used. The selection of a particular metallurgical bond forming deposit, or bonding agent, is influenced by the nature of the matrix/binder that is to be used for mounting the composite abrasive particles. Where the matrix/binder is a metal, it is generally preferred that such deposits or bonding agents form alloys with the metallic matrix/binder. The matrix/binder will preferably wet the surface of the composite particle to which it is exposed. This promotes the retention of the abrasive core particle in the finished article. These materials are conveniently described as xe2x80x9cmatrix/binderxe2x80x9d materials because they generally serve both functions. That is, they are generally the continuous phase in the finished article, which is composed of a plurality of composite abrasive grains. And they serve to hold the multi-grains together in a single coherent article of a desired predetermined shape.
The composite abrasive particles can be formed into useful articles employing a wide variety of procedures as is generally known with regard to powdered metals. Such procedures include, for example, sintering, brazing, casting, thermal or plasma spraying, wire arc transfer, D-gun, or the like. These particles can also, for example, be incorporated into electro formed abrasive products, and the like.
Other objects, advantages, and novel features of the present invention will become apparent from the following detailed description of the invention.